The present invention relates to a laminated polyester film. More particularly, it relates to a laminated polyester film which is not impaired in its transparency and has excellent antistatic properties even when it is produced by the so-called in-line coating method.
Biaxially stretched polyester films have many excellent properties such as flatness, chemical resistance, mechanical strength, etc., but have the disadvantage that they tend to be electrically charged.
As means for preventing static charging, it is generally practiced to incorporate a low-molecular-weight surfactant type compound as an antistatic agent in a polyester resin or to coat said compound on the surface of a polyester film. However, the low-molecular-weight antistatic agents unfavorably tend to bloom.
High-molecular-weight antistatic agents such as sodium polystyrenesulfonate are known as the antistatic agents proof against blooming. However, in case of using sodium polystyrenesulfonate, when the so-called in-line coating method, that is, a method in which the surface of a polyester film is coated and then the coated polyester film is stretched, is used for forming a laminated polyester film, the coating layer may become discontinuous depending on the working conditions, making it unable to obtain the desired antistatic effect. Further, the coating layer may have many cracks to cause blushing of the film, so that this is unsuited for use where transparency is required.
The present inventors have found that by using a specific antistatic agent, a laminated polyester film which is not impaired in its transparency and has excellent antistatic properties even when produced by the so-called in-line coating method can be obtained. The present invention was attained on the basis of this finding.